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Abstract

Polymer solar cells are attracting attention as inexpensive versatile devices for generating electricity from sunlight. However, relatively low efficiencies are currently hindering their widespread application. The typically low efficiencies arise because of the complex physics within these devices. In particular, photons must first be absorbed to create a mobile excited state, or exciton. Then this exciton must dissociate into free charge at the interface between an acceptor and a donor polymer, and finally, the free charge must traverse the polymer solar cell to the correct electrodes. Mathematical and computer models play an important role in understanding the physics of these devices and ultimately allow us to tailor the internal structure and material properties to optimize device performance. A brief review of polymer solar cells is presented, with particular emphasis on their nanoscale architecture, before the chapter turns its attention to the simulations and models that can predict their behavior.

Introduction

More energy is incident upon the Earth in one hour than is consumed by the world's population in an entire year. Harvesting just a small fraction of this solar energy could provide the solution to our long-term energy needs. However, first it is necessary for this solar energy to be captured, converted, and stored in a cost-effective fashion (Lewis, 2007). Solar cells are a promising method of both capturing this energy, and converting it directly into electrical energy. While this still leaves the need for energy storage, in order to provide electrical power during times of darkness, there are still many benefits to using solar cells. Firstly, solar cells could be used to complement other technologies which do not rely on sunlight. For example, times of low sunlight and darkness often coincide with instances of high winds and rainfall which could provide wind and hydroelectric power, respectively (Hinrichs & Kleinbach, 2006). Secondly, our current fossil-fuel based methods of generating energy are having a devastating impact on our environment. Ice sheet disintegration, sea level rise, and other adverse consequences of global warming, have recently emphasized our need for developing inexpensive alternative energy sources (Kerr, 2007). Furthermore, due to the portable nature of solar technology, these devices will especially benefit isolated communities in the developing world, whose fragile existence is most likely to be affected by global warming. However, the need for solar cells which can generate electricity as inexpensively as fossil-fuel based technologies are required before this energy source can provide a practical long-term alternative.

Solar cells based on silicon are currently the dominant photovoltaic technology. Silicon solar cells are reliable and highly efficient at converting solar energy into electrical energy. Combined with the natural abundance of silicon, this has made silicon solar cells a popular choice since the first p-n junction devices where fabricated in the 1950's (Cummerow, 1954). However, the high cost of silicon, especially crystalline silicon which is the most effective material, has limited the societal impact of solar cell technology and led to an interest in alternative materials (Shaheen, 2005). Thin film inorganic materials, such as amorphous silicon, have also attracted interest but high processing temperatures can make all inorganic solar cells prohibitively expensive. That said, recent advances in solar concentration could decrease the amount of solar cell material required and decrease the costs of silicon photovoltaics, while providing novel features such as partially transparency devices with non-planar geometries (Currie, 2008). In terms of photovoltaic research, however, there is still an impetus towards finding alternative materials and developing new devices.

In recent years, organic materials have emerged as a possible alternative to traditional inorganic semiconductor solar cells. Organic photovoltaic cells are typically produced from either small-molecular-weight films or polymer films. One of the main advantages of organic materials is their high optical absorption which results in solar cell thicknesses on the order of 100 nm; a thousand times thinner than silicon-based solar cell and ten times thinner than inorganic thin film cells (Kietzke, 2007). The use of organic materials can also significantly reduce the costs of production through solution processing and continuous deposition techniques. In other words, less of these cheaper materials can be deposited at lower temperatures and over relatively large areas. The cost of producing organic solar cells, therefore, is significantly less than inorganic cells. However, organic solar cells are inefficient at converting solar energy into electrical energy, with power conversion efficiencies now exceeding 10% in the laboratory. This means that while the cost of polymer solar cells is small, their inefficiencies limit the “cost per watt” of these devices. In order for polymer solar cells to be economically viable the cost per watt of energy would have to be reduced, requiring an increase in efficiencies without driving up costs.

Exciton: Upon photoexcitation in polymer materials the electron is excited away from its atom, leaving behind a hole. However, the low permittivity in polymers means there is a high coulomb attraction between the electron and hole, and they move through the polymer material as a combined excited state or exciton.

Donor: A polymer material with a low ionization potential making it easy to remove an electron from this material.

Photoexcitation: The excitation of an atom, to allow an electron to move away from its atom, in response to the absorption of a photon.

Polymer: A long thin macromolecule. Conducting polymers used in organic solar cells are stiffer conjugated molecules.

Bilayer: A polymer solar cell which consists of a layer of donor and a layer of acceptor material.